Method for improving the air release of a lubricating oil in a hydraulic system

ABSTRACT

The invention provides a method for improving the air release of a lubricating oil in a hydraulic system, said method comprising supplying the lubricating oil to the hydraulic system. The air release is measured according to ASTM D3427.

FIELD OF THE INVENTION

The invention relates to a method for improving the air release of a lubricating oil in a hydraulic system.

BACKGROUND OF THE INVENTION

Lubricating oils are required in hydraulic systems to protect, lubricate and help transmit power. Current trends in the design of hydraulic systems are towards smaller reservoirs, low oil residence times and high power output. These changes are leading to increased problems of air entrainment. Air entrainment is a phenomenon wherein air bubbles (typically having a diameter of less than 1 mm) are dispersed throughout the lubricating oil. Entrained air can be distinguished from free air (a pocket of air trapped in part of the system), from dissolved air (lubricating oils may contain between 6 and 12 percent by volume of dissolved air) and from foam (air bubbles typically greater than 1 mm in diameter that congregate on the surface of the oil). Air entrainment can have a number of negative consequences including loss of lubricity, possible oxidation of the lubricating oil, noisy operation, lower efficiency and higher oil temperatures.

The air entrainment properties of a lubricating oil are typically measured using the ASTM D3427 air release test. This test measures the time needed for air entrained in the oil to reduce in volume to 0.2% under the test conditions and at the specified temperature.

The present inventors have sought to improve the air entrainment properties of lubricating oils that are used in hydraulic systems.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a method for improving the air release of a lubricating oil in a hydraulic system, said method comprising supplying the lubricating oil to the hydraulic system;

-   wherein the lubricating oil comprises at least 90% by mass, based     upon the mass of the lubricating oil, of base oil, and wherein at     least 70% by mass, based upon the mass of the base oil, is GTL base     oil; -   wherein the viscosity of the GTL base oil at 100° C. is from 2 to 20     cSt; -   wherein the lubricating oil comprises less than 10% by mass, based     upon the mass of the lubricating oil, of additives; -   wherein the viscosity of the lubricating oil at 40° C. is from 20 to     100 cSt; and -   wherein air release is measured according to ASTM D3427 and is     improved as compared to air release achieved using a lubricating oil     which comprises less than 70% by mass of GTL base oil.

The inventors have found that improved air release is achieved when the base oil in the lubricating oil is predominantly GTL base oil.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for improving the air release of a lubricating oil in a hydraulic system. In a hydraulic system the lubricating oil is used not only to lubricate the machinery but also to transmit pressure. Air entrainment can be a particular issue in hydraulic systems, causing spongy or erratic operation of the hydraulics.

The air release is measured according to ASTM D3427 (Version 14a, 2015). Compressed air is blown through the lubricating oil, which has been heated to a temperature of 50° C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is recorded as the air release time. A desirable air release value is typically less than 3 minutes, preferably less than 60 seconds and most preferably less than 20 seconds.

The air release is improved compared to air release achieved using a lubricating oil which comprises less than 70% by mass of GTL base oil. The comparison is between substantially equivalent lubricating oils wherein the only difference is in the amount of GTL base oil that is present. For example, the comparison should be between lubricating oils that are of the same viscosity and that contain the same additives. In the comparative lubricating oil there will be a greater quantity of non-GTL base oil. The non-GTL base oil is, for example, a base oil from Group I, II or III of the API base oil categories. By incorporating at least 70% by mass of GTL base oil the inventors have observed improved air release times as compared to lubricating oils having less than 70% by mass of GTL base oil (and wherein the GTL base oil has been replaced by a Group I, II or III base oil).

The lubricating oil is supplied to the hydraulic system using standard methods.

The lubricating oil comprises at least 90% by mass, based upon the mass of the lubricating oil, of base oil. At least 70% by mass, based upon the mass of the base oil, is GTL base oil. Preferably at least 75% by mass is GTL base oil. GTL base oils are synthesised by the Fischer-Tropsch method of converting natural gas to liquid fuel. They have very low sulphur content and aromatic content compared with mineral oil base oils refined from crude oil and have a very high paraffin constituent ratio. Up to 30% by mass (and preferably up to 25% by mass), based upon the mass of the base oil, may be another type of base oil, including conventional base oils chosen from Groups I, II, and III of the API (American Petroleum Institute) base oil categories. In one embodiment of the invention, the base oil comprises at least 10% by mass of a base oil chosen from Group I, Group II or Group III.

The kinematic viscosity of the GTL base oil at 100° C. is from 2 to 20 cSt, preferably from 3 to 15 cSt and more preferably from 3 to 10 cSt. The viscosity is suitably measured using ASTM D445.

The lubricating oil comprises less than 10% by mass, based upon the mass of the lubricating oil, of additives.

Preferably the amount of additives is less than 5% by mass. Preferably the amount of additives is at least 0.5% by mass. The additives may include antioxidants, antiwear additives, demulsifiers, emulsifiers, rust and corrosion inhibitors, VI improvers and/or friction modifiers. The additives may be supplied as additive packages, e.g. an ashless additive package or a zinc-based additive package.

The kinematic viscosity of the lubricating oil at 40° C. is from 20 to 100 cSt, preferably from 25 to 80 cSt. The kinematic viscosity is suitably measured using ASTM D445 (ASTM D7042). This range of viscosities provides lubricating oils suitable for use in hydraulic systems.

The invention is further explained in detail below by means of examples, but the invention is in no way limited by these examples.

EXAMPLE 1

Eleven different base oil blends were prepared using combinations of three base oils:

-   -   1) XHVI 8 (a Fischer-Tropsch derived oil available from Shell         having a kinematic viscosity at 100° C. of approximately 8 cSt)     -   2) Yubase 8 (a Group III base oil from SK Lubricants having a         kinematic viscosity at 100° C. of approximately 8 cSt)     -   3) Chevron 220 R (a Group II base oil from Chevron having a         kinematic viscosity at 100° C. of approximately 8 cSt)         The air release time of the base oil blends was tested using the         ASTM D3427 method at 50° C. Table 1 gives the amount of each         base oil (in weight % based upon the total weight of the blend)         present in each blend and the air release time at 50° C. The         blends in Table 1 are all ISO 46.

TABLE 1 Air release XHVI Yubase Chevron time at 50° C. 8 8 220 R (seconds) Blend 1 100 0 0 5 Blend 2 75 25 0 5 Blend 3 66.7 33.3 0 30 Blend 4 50 50 0 40 Blend 5 25 75 0 60 Blend 6 0 100 0 77 Blend 7 0 0 100 61 Blend 8 50 0 50 46 Blend 9 90 0 10 5 Blend 10 80 0 20 5 Blend 11 75 0 25 15

The results show very fast air release for the blends having from 75 to 100 wt % GTL (Blends 1, 2, 9, 10 and 11). All the other blends, having less than 70% GTL, have significantly slower air release times.

EXAMPLE 2

Air release times are typically considered to get worse with the addition of additives. To demonstrate that fully formulated hydraulic oils according to the invention are resistant to changes in air release, blends were prepared using XHVI 8 (a Fischer-Tropsch derived oil available from Shell having a viscosity at 100° C. of approximately 8 cSt) and different amounts of either of the following:

1) Additive Package 1 (ashless additive package)

2) Additive Package 2 (zinc based additive package)

Table 2 gives the blend compositions (in weight % based upon the total weight of the blend) for each of the additive packages with XHVI 8 and the corresponding air release times:

TABLE 2 Air Release Additive Additive XHVI Time at 50° C. Package 1 Package 2 8 (seconds) Blend 1 0 0 100 5 Blend 12 1 0 99 10 Blend 13 2.5 0 97.5 10 Blend 14 5 0 95 10 Blend 15 0 1 99 10 Blend 16 0 2 98 10 Blend 17 0 3 97 10 Blend 18 0 4 96 10 Blend 19 0 5 95 10 Blend 20 0 10 90 10

The results show that the addition of additive packages (either ashless or zinc based) does not significantly affect the air release properties of the lubricating 

1. A method for improving the air release of a lubricating oil in a hydraulic system, said method comprising supplying the lubricating oil to the hydraulic system; wherein the lubricating oil comprises at least 90% by mass, based upon the mass of the lubricating oil, of base oil, and wherein at least 70% by mass, based upon the mass of the base oil, is GTL base oil; wherein the viscosity of the GTL base oil at 100° C. is from 2 to 20 cSt; wherein the lubricating oil comprises less than 10% by mass, based upon the mass of the lubricating oil, of additives; wherein the viscosity of the lubricating oil at 40° C. is from 20 to 100 cSt; and wherein air release is measured according to ASTM D3427 and is improved as compared to air release achieved using a lubricating oil which comprises less than 70% by mass of GTL base oil.
 2. The method according to claim 1, wherein the base oil comprises at least 10% by mass of a base oil chosen from Group I, Group II or Group III.
 3. The method according to claim 1, wherein the viscosity of the GTL base oil at 100° C. is from 3 to 10 cSt.
 4. The method according to claim 1, wherein the lubricating oil comprises less than 5% by mass, based upon the mass of the lubricating oil, of additives.
 5. The method according to claim 1, wherein the lubricating oil comprises at least 0.5% by mass, based upon the mass of the lubricating oil, of additives.
 6. The method according to claim 1, wherein the viscosity of the lubricating oil at 40° C. is from 25 to 80 cSt. 